Compared with silicon (Si), silicon carbide (SiC) has superior properties which include a band gap that is about three times greater, an insulation breakdown electric field strength that is about 10 times greater, and a thermal conductivity that is about 3 times higher. SiC therefore holds considerable promise for applications to power devices, high-frequency devices, and high-temperature operation devices and the like. As a result, SiC epitaxial wafers are starting to be being used for semiconductor devices.
SiC epitaxial wafers are manufactured using SiC single crystal substrates, which are processed from SiC bulk single crystals prepared by a sublimation method or the like for use as substrates for forming SiC epitaxial films. A SiC epitaxial wafer is obtained by growing a SiC epitaxial layer on a SiC single crystal substrate using a chemical vapor deposition (CVD) method. SiC epitaxial layers function as the active regions of SiC semiconductor devices.
The performance of a SiC semiconductor device varies depending on the state of the grown SiC epitaxial layer. The manufacture of SiC epitaxial wafers having uniform quality and minimal defects is desirable.
For example, Non-Patent Document 1 discloses a method in which HCl gas is introduced together with the raw material gases during SiC epitaxial growth. Non-Patent Document 2 discloses a method in which trichlorosilane (siHCl3) is used as a raw material gas during SiC epitaxial growth.
Non-Patent Documents 1 and 2 discloses that by using a gas composed of molecules containing a Cl atom within the molecular structure (hereafter referred to as a “Cl-based gas”) during SiC epitaxial growth, nucleus growth of Si in the gas phase can be suppressed, and the formation of Si liquid droplets on the wafer surface can also be suppressed. Examples of the Cl-based gas include HCl and chlorosilane-based compounds.
Si nuclei formed in the gas phase and Si liquid droplets on the wafer surface can become defect origins during epitaxial growth. Accordingly, in Non-Patent Documents 1 and 2, a Cl-based gas is used to manufacture SiC epitaxial wafers having minimal defects. Non-Patent Documents 1 and 2 also disclose that by using a Cl-based gas, the growth rate of the SiC epitaxial growth can be increased.
Patent Document 1 discloses the use of ammonia or the like as the dopant gas instead of nitrogen gas. In nitrogen gas, the energy of a triple bond between the N atoms is very high, and controlling the thermal decomposition process is difficult. As a result, when nitrogen gas is supplied to the substrate as an active species that contributes to doping, achieving a uniform distribution for the nitrogen partial pressure across the substrate surface is difficult. Accordingly, by using a dopant such as ammonia that lacks a triple bond, the uniformity of the carrier density for the SiC epitaxial film can be enhanced. Further, Patent Document 1 also discloses that by decomposing the ammonia or the like in advance in a preheating step, the uniformity of the carrier density can be enhanced even further.